Posterior spinal implants serve to correct sagittal spinal deformities (kyphosis, spondylolisthesis) or to act as stabilizers when there is no correction (degenerative instabilities). As a rule they comprise a head having a slot for receiving a support rod, an anchoring segment connected to said head and a tightening screw positioned to press on a support bar in the slot of the head. A stabilizing element may also be present on the head.
The anchoring segment may be a screw or hook. The screw may be fastened directly above the pedicle of the vertebral body. The hook may be suspended from the vertebra. In relation to the number of vertebrae to be corrected or stabilized, several screws or hooks firmly joined to the support rod and thus fixed in place are required for spinal correction or stabilization.
In most known spinal implants, the tightening screw is directly screwed into the head after the support rod has been inserted into the slot. Such practice has a number of drawbacks.
Because the U-shaped slot receives the support rod, the end of the head consists merely of two tulip-shaped jaws. Thus, the tightening screw rests only in the threads of these jaws. When the tightening screw is screwed in under pressure, the surgeon has trouble sensing whether the screw is properly engaging the thread. Furthermore, to prevent the slot from widening, a stabilizing element must be added.
French patent document FR-A 2,624,720 discloses an implant wherein the two legs of the head forming a U-shaped slot have an external thread which may receive a screw-on cap through a central threaded part of which an actual fixation part can be screwed in. However, the manufacture of this known device is difficult. Moreover, it leads to an undesired enlargement of the head with respect to both height and diameter.